Van Marum in 1785 observed that there is a particular smell associated with electrical discharges in air. In 1801 Cruickshank noticed and confirmed the same unusual smell in an electrolytic cell for oxygen. Finally, in 1840 Schonbein a Greek scientist reproduced and identified a new gas he smelled, and in Greek "I smell" means Ozo, thus he named it as Ozone.
Ozone is a high energy state of oxygen occurring naturally in traces in the rural atmosphere and the sea-side air from the action of ultraviolet light on oxygen. It is a powerful oxidizing agent and is used in many domestic and industrial applications for wet oxidation of organics. The use of ozone is increasing throughout the world and specially ozone use has increased in the US by 400% in last five years. At this rate of growth (i.e. 30%/year) the use of ozone will increase to 2,1000% by the end of the century. Ozone's use in water treatment is extremely beneficial for the environment and provides an excellent water quality. There are no other hazardous chemicals involved and it itself reverses back to oxygen.
While oxygen may be converted into ozone by chemical, electrical and optical means, typically ozonized oxygen is produced in a special type of apparatus called an ozonizer or ozone generator. Nearly all ozone generators in use today, produce ozone by passing air or oxygen through a corona field. These ozone generators utilize the dielectric constant of various materials along with the principles of electrical capacitors at high voltages to produce a corona (or high intensity electric field) in the air space between the two electrodes of a capacitor. When oxygen passes through this highly charged corona, ozone is formed.
Ozone is an allotrope of oxygen, i.e oxygen atoms are the building blocks. The oxygen molecule has two atoms of oxygen while the ozone molecule has three atoms of oxygen. In the forward reaction, oxygen molecules convert to ozone. In the reverse reaction the ozone molecules convert back to oxygen. This conversion of oxygen into ozone is represented in the following equilibrium: EQU 3O.sub.2 +69Kcals.revreaction.2 O.sub.3
According to the above equation the ozone production reaction is endothermic in nature. Hence the proportion of ozone in equilibrium with oxygen is small at low temperatures. The proportion of ozone increases with an increase in temperature. But at very high temperatures, the reverse reaction also increases and there is a greater decomposition of ozone back to oxygen. Both the forward and reverse reactions compete with each other and as a final result the forward reaction is faster than the reverse reaction by only 4% at room temperature (i.e. the corona helps convert 100% into ozone while the same corona converts back 96% to oxygen). Typically, ozone generators, when using dry air produce about 2% ozone with the balance being oxygen and nitrogen (i.e., the constituents of air). However, when 100% dry oxygen is used, ozone generators may produce ozone in concentrations of up to 4%. Consequently, the ozone generators currently in use today can produce only limited concentrations of ozone, typically on the order of 4%.
The Corona
The conversion of oxygen into ozone is an endothermic reaction, thus requiting energy for the conversion of oxygen to ozone. However, it is well known that this energy must be of a form other than heat energy since raising the temperature of the reactant, oxygen, will result in an increase in the decomposition rate of the desired ozone; i.e., the reverse reaction rate increases with increasing temperature. Most ozone generators in use today use a "corona discharge", i.e., a high intensity electric field with no glow discharge (visible ionization of the gas), as the most convenient source of energy for this purpose.
A corona discharge is produced between two conductors charged to a high electric potential (typically from 7,000 to 12,000 volts) and separated by an air gap and dielectric. The voltage needed to establish the high electric potential may be generated by any suitable source such as a transformer, induction coil, or a voltage multiplier circuit. A corona discharge imparts minimum heat and therefor does not raise the temperature of the oxygen, thus reducing the decomposition rate of the ozone. However, it is also known that an electric spark or arc is undesirable as this would heat the ozone and cause its decomposition almost as fast as it is formed. Moreover, arcing would lead to formation of nitrogen oxides when air is used as a feed gas, thus chemically consuming the ozone as soon as it is formed.
Spacing of the two conductors becomes critical as the breakdown potential of the air gap and dielectric is approached. Arcing will occur at the breakdown potential resulting in local heating of the ozone and it consequent decomposition. Further, as arcing is essentially a short circuit of the high voltage source, it will destroy the homogeneity of the electric field thus significantly impacting the efficiency of the ozone generator.
Electrodes
At present, nearly all ozone generating technologies utilize electrodes to produce a corona. These electrodes are usually coaxial robes and the gap between the electrodes is shielded by a dielectric substrate. Upon connecting electrodes to a high potential difference, a corona is produced which excites the oxygen to convert to ozone.
The production of ozone in a direct current corona has been investigated by numerous researchers. The basic mechanisms of the direct current corona have been documented by L. B. Loeb in two texts: Basic Processes of Gaseous Electronics, University of California Press, Berkeley and Los Angeles, 1955 (Second Edition 1961), and Electrical Coronas-Their Basic Physical Mechanisms, University of California Press, Berkeley and Los Angeles, 1965. However, the quantitative evaluation of ozone production in coronas is not discussed in either of these references.
Ozone production from pointed electrodes and wire electrodes have been reported in the literature (Goldman M., Lecuiller M. and Palierne M., Gaseous Discharges III, Pergamon Press, 1982 p 329; Peyrous R. and Lacase C., Ozone Science and Engineering 8, pp 107-128, 1986). In much of this work there is general disagreement about the efficiency of ozone production in positive or negative coronas, and the role of electrode materials in improving ozone production by catalytic processes.
The production of ozone in large quantities by corona discharges using a primary and secondary corona process in confined tabulation with forced air flow, has been described by Imris in U.S. Pat. No. 4,062,748. The apparatus shown required the forced circulation of air or oxygen through tubular arrays, to produce ozone outputs at the level of grams per hour. Corbeil, Canadian Patent No. 935,784, issued Oct. 23, 1973, describes a gas treatment apparatus in which ozone is produced by a direct current corona between a sharp point and a section of spherical gauze as the counter electrode. The device is enclosed in a robe of insulating material and disperses ozone with the aid of the associated ion wind. No measurements or estimates of the amount of ozone produced are included in the patent.
From the work of Loeb, it is clear that in the silent DC corona there can be marked differences in the effective temperature of the corona, depending on the local current density in the discharge. As the corona degenerates to a glow discharge, and finally to an arc, this temperature continually increases, thus further increasing the reverse reaction rate (i.e., decomposition of ozone into oxygen). It is known that in this are mode, the production of ozone is small, as ozone produced by dissociation of oxygen by ion impact, is quickly reverted to oxygen at the elevated temperature of the arc. Even in silent corona discharges it is important to control the local temperature in the discharge, so that ozone is not rapidly converted to oxygen after initial formation. Consequently, Loeb teaches that: the discharge from sharp points is to be avoided if the ozone is to be produced efficiently in low current corona discharges.
Although Loeb has shown that sharp points and knife edges as used to generate a corona exhibit high local temperatures, these elements continue to be researched and used despite their low efficiencies. For example, Satoh, et al., U.S. Pat. No. 4,507,266, issued Mar. 26, 1985, discloses an electrode structure having a plurality of edges. Glow discharge is generated between edge portions associated with the cathode and anode surfaces of his device. Such sharp edges result in areas of local high temperature due to glow discharge or arcing. McBlain, U.S. Pat. No. 1,588,976, and Napier, U.S. Pat. No. 2,155,675 teach using corrugated wire mesh electrodes. Mesh electrodes have been frequently corrugated to act as spacers between adjacent dielectric shields to form air gaps where the corona discharge and ozonization takes place. However, such a construction can create hot spots since the electrode is spaced in varying degrees from its complimentary electrode, and the air gap is somewhat encumbered by the presence of the electrode material in the space. Hot spots and sparking sometimes result when the electrode comprises metal projections struck from a sheet metal plate as, for example, disclosed in Leggett, U.S. Pat. No. 1,010,777.
Another cathode/anode geometry, the tube-in-tube (concentric tubular) arrangement avoids using sharp edges to generate a corona and works on the theory that as oxygen passes between the tubular electrodes, it is provided with an extended exposure to an albeit lower energy corona field, thus maximizing the production of ozone. However, this arrangement also permits the reverse reaction of ozone decomposition to establish itself as well. As noted supra, at room temperature, the forward reaction is slightly faster than the reverse reaction resulting in only a 4% concentration of ozone at equilibrium. Thus, the effect of the tubular electrodes is to permit the equilibrium to become established. Beitzel, Canadian patent No. 920,088, issued Jan. 30, 1973, discloses a tubular electrode structure for an ozone generator including a dielectric tube with an electrode on the inner surface of the tube and an electrode on the outer surface of the tube.
Other electrode configurations are also possible. For example, plate type electrodes are disclosed in Schaefer, Canadian patent No. 920,979, issued Feb. 13, 1973, and in Pavel, Canadian Patent No. 1,090,293, issued Nov. 25, 1980.
Size
While ozone generators of the background art seek to maximize the surface area of the electrodes, many of the designs also seek to minimize the increase in size of the ozone generating unit as a consequence of increasing the surface area of the electrodes. Consequently, many ozone generators arrange a number of dielectric shields in cylindrical form in concentric spaced relationship in order to provide maximum electrode surface area while resulting in a minimum increase in size. The spacing between the concentric tubes may be affected by either corrugating the electrode or by striking tongues from the tube walls, as for example, the configuration disclosed by Blackmore et al. in U.S. Pat. No. 743,433. However, such electrode constructions have the aforementioned disadvantages of low ozone concentrations (net 4%) due to establishment of the reverse decomposition reaction, of creating hot spots, and creating interference with the free flow of air to be ozonized through the partially obstructed spaces between the dielectric tubes. A similarly troublesome attempt at efficiently utilizing the space within an ozone generating unit is disclosed in McBlain, U.S. Pat. No. 1,588,976, which teaches forming a spiral roll of successive layers of dielectric and electrode-forming sheets.
Accordingly, there is a need for an ozone generator that is not limited to the net 4% concentration of produced ozone, one that is compact in size and does not create hot spots.
Cooling
Typically, glass or similar material is used as the dielectric shield. However, these materials can be readily cracked or punctured due to moisture or other contaminants in the oxygen or oxygen-containing gas mixtures resulting in an uneven electric field flux onto the dielectric and, in severe cases, arcing. This results in local high temperatures which may thermally crack or puncture the dielectric. Alternately, the high energy corona also creates a substantial amount of stress in the shield material which could also crack the shield. Accordingly, as any heat generated favors the reverse reaction and is potentially destructive to the ozone generator device, it must be dissipated from the corona zone. Such heat dissipation requires an elaborate cooling system.
Typically, a portion of the heat produced by the ozonation process is dissipated by enlarging the electrode surface area with respect to the length of the corona-filled air gap, as exemplified by the tubular electrode arrangement. However, this creates problems of warping with resulting short circuits. Others have sought to cool the electrodes by passing a refrigerant such as water or brine through and/or over them. Aqueous refrigerants cannot be used within the corona area and are, therefore, confined to use within the electrode structure. This calls for enlarging the physical size of the cooled electrode and requires that one side of the electrical circuit be grounded. The serious disadvantage to grounding, however, is that it prevents the use of a center-tapped grounded transformer in the equipment involved to reduce the magnitude of the voltages with respect to ground.
The above mentioned dielectric shield materials can be readily cracked or punctured when excessive stresses are applied to them by hot spots or wide variations in temperature of the air moving over different portions over them. This cracking or puncturing of the dielectric shield will destroy the insulating qualities of these dielectrics and result in arcing and the eventual complete destruction of the dielectric shield. Hot spots can be caused by an unequal distribution of the electric field due to variations in the spacing between the electrodes. Wide extremes of temperature of the air moving over the dielectric shield can be caused by uncontrolled air inlet temperatures during ozonation, and electrodes with large surface areas. While it has been proposed to pre-cool the air to be ozonized in an ozone generating device, as disclosed by Fleck in U.S. Pat. No. 3,024,18, such pre-cooling has not been commercially utilized to any significant extent because it increases the possibility of undesired temperature ranges of the air flowing over the dielectric shields. This temperature gradient may result in sufficient thermal stress so as to crack the dielectric. Thus, most commercial ozone generating units utilize grounded water-cooled jackets surrounding the outermost electrodes resulting in expensive, bulky equipment which in many cases does not adequately cool those portions of the ozone generating unit not immediately positioned near the cooling jackets.
Accordingly, there is a need for an ozone apparatus having integral cooling of the apparatus, cooling of the inlet oxygen without creating wide temperature variations or destruction of dielectric, or smaller electrode surface areas resulting in less thermal stress.
Commercial ozonizers currently used today are typified by the Siemen's Ozonizer, Brodie's Ozonizer and the Siemen's and Halske Ozonizer. All of these ozonizers employ either a concentric tube or parallel plate electrode design having large surface areas, feed the air or oxygen in at atmospheric pressure, and either air cool or water cool the electrodes. For the Siemen's and Halske Ozonizer, the one most frequently used in industry, the supply voltage ranges between 7 KV to 12 KV @500 Hz. Production is at the rate of 50 grams of ozone per kw-hr at a concentration of 2.5 grams of ozone per cubic meter. By comparison, from the heat of reaction (-69 Kcals/mole), the theoretical energy for the production of one gram mole of ozone from oxygen corresponds to a theoretical formation of 1200 grams ozone per kw-hr. Thus, even when operating at low concentrations, the best energy efficiencies exhibited by commercial ozonizers are on the order of 5 to 15%.
Currently available ozone generators and ozone generator technology are thermodynamically inconsistent with the efficient production of ozone. Further, commercial ozonizers exhibit unacceptably low energy efficiencies. Accordingly, there is a long felt need for an ozone generator or ozone generator system whose structure and operating conditions thermodynamically favor the high yield, high volume energy efficient production of ozone.
Water Treatment
On average, an adult should consume approximately two liters of water a day; the equivalent of eight 8-ounce glasses of water. In the process of such consumption, the average person is also consuming those chemical disinfectants used to make the water "safe" to drink. These include the use of chloramines and chlorine disinfectants. Aside from imparting an offensive odor and taste to the water, scientists are now warning that chlorine combines with various dissolved organic compounds to form dangerous chlorohydrocarbons such as thrichloromethane (TCA), thrichloroethane (TCE) and chloroform. These chlorohydrocarbons have been determined to be potential cancer causing agents.
There is a growing recognition that traditional water purification measures--such as ultraviolet irradiation, Chlorine disinfection or halogenated resins--are not adequate to prevent illness caused by some virulent "newcomers." A good example is the Crypto sporidium oocyst (the encysted form of a bi-flagellated protozoan) which is responsible for causing crytosporidiosis in humans. This oocyst can survive for a long time in the environment and cannot be killed by traditional methods. Other microbial or contagious vectors include giardia lamblia (an enteric protozoan which causes a diarrheal illness), hepatitis A, shigella sonei and E. coliform bacteria. More than 110 types of human viruses--including the entoviruses, hepatitis A virus, Norwalk virus, rotavirus and adenovirus--are major causes of waterborne disease.
In 1985, seventy five thousand pneumonia-like illnesses and 11,250 deaths were attributed to Legionnaires' disease resulting from exposure to legionella. Legionella can infect by inhalation of water vapor from cooling towers, evaporative condensers, hot water tanks, lakes, ponds, streams, puddles, swimming pools, spas or whirlpools and dental units. One study found legionella in 14% of the ground water tested.
Ozone has been extensively investigated as an alternative to chlorination and is now considered the preferred choice for water purification. Ozone disinfects water 5,000 times more rapidly than chlorine, destroying viruses and bacteria in the process. It is also a less expensive process. Over 3,300 European cities treat their drinking water with ozone. Likewise, thirty U.S. cities ozonate their water supply, including Los Angeles which has installed the largest ozone treatment plant in the U.S.
Ozone has been found to be effective against Crypto sporidium, legionella, host protozoa and other biocontaminants. Dosage requirements and contact time will vary depending on the water characteristics, but common ranges are 1 to 10 mg/l of ozone for 15 minutes.
With ozone becoming fast recognized as the disinfectant/bioeradicant of choice in both drinking water and wastewater treatment, there will be a growing demand for ozone production. Applications will range from municipal treatment centers to point-of-use (POU) devices for treating water in the home or office. However, since ozone is an unstable gas and breaks down into molecular oxygen relatively quickly, it must be generated in situ. Accordingly, there is a need for a high yield, high efficiency ozone generator device for the treatment of drinking water and waste water, adaptable for use in from large scale drinking water and waste water in situ ozonation facilities, to small POU in situ applications.
Air Treatment
Sulfur dioxides (SO.sub.x) and nitrogen dioxides (NO.sub.x) are criteria pollutants under the U.S. Clean Air Act. These compounds are known to contribute to the formation of acid rains which pollute streams and lakes, destroy buildings and kill forests. Consequently, much research has gone into the removal of SO.sub.x and NO.sub.x from flue gas. Methods currently used include:
1) Lime, limestone and dual alkali processes--Form calcium sulfites and sulfates as byproduct. Land fillable disposal. PA1 2) Sodium or ammonia solution--Process uses sodium or ammonia hydroxide or carbonate. Resulting sodium sulfite and nitrites require treatment. PA1 3) Wellman-Lord Process using aqueous sulfite--Liquid SO.sub.2, liquid SO.sub.3, sulfuric acid or elemental sulfur are byproducts. PA1 4) Citrate--Process using water enhanced by citric acid and sodium hydroxide or carbonate. PA1 5) Magnesium Oxide--Process using magnesium oxide. Liquid SO.sub.2, liquid SO.sub.3, sulfuric acid or elemental sulfur are byproducts. PA1 6) Dry Sodium Bicarbonate powder injected upstream of a baghouse. PA1 a cold trap whereby the ozone is condensed out; PA1 a molecular membrane filter of a pore size to permit the oxygen to pass through but retain the ozone; PA1 a vortex cooler (Maxwell demon) wherein the heavier ozone molecules are tapped off from the center of the cooler, and PA1 chemical separation, such as immediate consumption of the ozone, or reversible sequestration of the oxygen, e.g. by a barium-barium oxide pellet system. The oxygen is collected, cleaned and dried if needed, repressurized and recirculated to be used as feed gas.
These treatment technologies are either very complicated and capital intensive (Wellman-Lord, titrate and ammonia processes) or create a waste which requires further treatment prior to disposal.
Ideally, a process for removing SO.sub.x and NO.sub.x would be simple and recycle the sulfur and nitrogen as a useable raw material. Accordingly, there is a need for an air treatment process for the removal of SO.sub.x and NO.sub.x from flue gas that will allow reclaiming the sulfur and the nitrogen.